This invention relates generally to the control of light power in an optical storage system, and in particular to a system and method for controlling laser power in a magnetic super-resolution magneto-optical storage system.
Data storage systems may store digital data using a plurality of different storage media. These include magnetic media, optical media, and magneto-optical (MO) media. Magneto-optical (MO) storage systems are advantageous in that the MO media may be written and rewritten multiple times and read using laser light. Data is written onto MO media in a magneto-optical storage system by impinging the laser light onto the MO medium at a power level that is sufficient to heat the MO medium to a predetermined temperature known as the Curie temperature, and a magnetic field is applied to the MO medium such that magnetic dipoles within the particular heated region of the medium align with the externally applied magnetic field with a polarity determined by the direction of the applied field. Depending upon the direction of the applied magnetic field, the magnetic dipoles in the region will store a magnetic representation of a binary "1" or a binary "0". Subsequently, data is read by impinging an incident laser light beam having a predetermined polarization onto the MO medium. The polarization of the light beam reflected from the MO medium is rotated relative to the polarization of the incident laser light either clockwise or counterclockwise from the incident polarization due to the direction of magnetization of the particular magnetic region from which the beam is reflected. This is known as the Kerr effect, and the reflected beam can be processed by an optical detection system to read the data stored in the illuminated region of the MO medium.
In order to accurately read or write data onto and from MO media, the power of the laser light that impinges on the MO media should be controlled. Too much laser power may damage the media, while too little power may not heat the media sufficiently to permit magnetic dipoles to be altered. The appropriate amount of laser power required for reading and writing data onto the MO media depends upon the media temperature, and is influenced by a number of factors such as media physical characteristics and operating conditions. These include the speed of rotation of the media, the temperature change of the media during operation, variations in media sensitivity with radius, and changes in recording and writing efficiency of the MO storage system. It is desirable to compensate for such factors in real time by determining the appropriate laser power level and controlling the laser power for different operating conditions.
In some MO storage systems that use a conventional MO storage medium, the continuous laser power can be adjusted according to the speed of rotation of the medium to write or erase a data track of a constant radial width. For a given power, it is known that the temperature of the storage medium will vary non-linearly with the rotational velocity of the medium. In such systems, the laser power is typically adjusted based upon the calculated or measured changes in the velocity of the storage media. The shortcoming of this approach is that it is not based on actual measurements of the medium itself, and it does not take into consideration factors other than velocity which influence the temperature of the storage medium.
Some MO media are formed on a metal substrate which has good thermal conductivity so that changes in the media temperature are more predictable. Changing laser power based only the rotational speed of the storage media may possibly be an acceptable approach for controlling laser power with such media. However, in order to reduce the overall costs of the MO media and the MO storage system, it is common to use less costly plastic substrates for the MO media. As compared to metal substrates, plastic substrates have poor thermal conductivity characteristics so that the temperature of the plastic media may experience greater fluctuations in temperature as portions of the media are heated. For this type of media, controlling laser power only as a function of media rotational speed does not consider ambient media temperature at the time of writing or reading and is not acceptable.
Moreover, a new type of MO media referred to as magnetic super resolution (MSR) storage media has been developed for use in magneto-optical storage systems. It utilizes an upper magnetic readout layer and a lower buried magnetic storage layer. The buried storage layer may be a conventional MO medium that is not accessible for reading until the upper readout layer is heated sufficiently with laser light to reach a predetermined read temperature at which an "aperture" is formed through which the buried layer can be viewed. This aperture enables the magnetic flux in the storage layer to be copied to the readout layer and be visible at the media surface.
With MSR storage media, the laser power applied to the MSR media during a read operation determines the temperature profile in the readout layer, and thus the aperture size. The power should be carefully controlled so that the aperture is maintained with a constant size which is independent of operating conditions such as media velocity and ambient media temperature. If the aperture is smaller than the magnetic region in the buried storage layer beneath it, the magnetic flux coupled to the upper readout layer may be insufficient to generate an adequate signal for readout. If the aperture is too large, the light will be reflected from adjacent magnetic regions and may be degraded because of intersymbol interference. The laser power applied to the MSR media should also be controlled during a write operation since the written magnetic region should be wide enough so that it is readable during a read operation if the incident laser light is slightly off-track. The laser power during writing of data should also be controlled so that adjacent regions are not inadvertently overwritten. The appropriate laser power for a reading or writing is determined by the temperature of the media, which is a function of a number of different factors and may vary substantially in real time. Controlling laser power on the basis of rotational velocity of the storage media, fails to take into consideration the actual time-varying operating conditions of the media and does not control laser power appropriately to actual conditions. Thus, some systems attempt to monitor the temperature of the storage media using temperature sensors adjacent the media, and control laser power in response to sensed changes in media temperature. However, accurately measuring the temperature of a rotating medium using temperature sensor is very difficult, particularly at the point of the read/write head.
Thus, there's a need for a system and method that affords real-time control of laser power for reading and writing of data in a magneto-optical storage system that utilizes MSR storage media, and which avoids the foregoing and other problems of known systems and methods. It is to these ends that the present invention is directed.